322 HEATS OF FOEMATION OF THE CYANOGEN SERIES. 



the same acid, with which it co-exists in the liquors. This 

 excess of energy exactly measures the tendency to produce the 

 inverse reaction, which, however, ceases as soon as the anhy- 

 drous hydracid contained in the liquor is saturated. 



But, on the contrary, the reaction could not be foreseen, as has 

 been supposed by various writers, 1 from the knowledge of the 

 quantity of heat disengaged in the dilution of the concentrated 

 acid, the bulk of which becomes a dilute acid. Not only is this 

 mode of prediction not justified in principle, since it makes no 

 distinction between the anhydrous acid and its hydrates in 

 solution, but it leads to conclusions which are quite contra- 

 dicted by experiment. For example, mercuric cyanide is still 

 decomposed in the cold by hydrochloric acid of a density I'lO 

 (which nearly corresponds to HC1 -f 7H 2 0) ; the dilution of 

 such a hydrochloric solution gives off only 1-7 Cal. Now, this 

 excess would have to be equal to + 6*0 for the reaction to be 

 reversed, according to the theory that we must reject ; i.e. that 

 the inversion is solely due to the heat of dilution taken in the 

 mass. This excess is so great that the dilution of even the most 

 concentrated hydrochloric acid could not make up for it. 



The greater number of reciprocal displacements give rise to 

 the same observations, the heat disengaged by the dilution of 

 concentrated acids or alkalis being scarcely ever sufficient to 

 supply the whole of the body in solution with the energy 

 necessary to reverse the chemical reaction ; whereas this energy 

 is, on the contrary, supplied by the hydration of the portion of 

 acid (or alkali) which existed in the liquor in a dissociated 

 state. 



7. But to return to mercuric cyanide. Theory indicates that 

 the displacement of hydrochloric acid by hydrocyanic acid in 

 mercuric chloride may be observed still more clearly if we 

 substitute an alkaline cyanide for the free hydrocyanic acid. In 

 fact, in this case, we shall get, besides, the difference of the 

 heats of neutralisation of both acids by the alkali. This is 

 confirmed by experiment. 



i[2KCN(l equiv. = 8 litres) + HgCl 2 (1 equiv. = 4 litres)] 



disengages + 16*7. 



J[2KC1(1 equiv. = 8 litres) -f- Hg(CN) a (1 equiv. = 4 litres)] 

 disengages -f 0. 



Now, calculation gives 

 (M-M 1 )-(M'-M' 1 ) = (13'6 -3) -(9-5 -15-5) = + 16-6, 



a result quite consistent with the above. Thus, the reality of a 

 double integral interchange between the bases and acids in 

 solution is fully established. This is one of the most glaring 

 cases in which the so-called saline thermo-neutrality which was 



1 " Annales de Chimie et de Physique," 5 s s6rie, torn. iv. p. 464. 



